Self-inspection topology design for battery energy storage

ABSTRACT

According to one embodiment, a BBU includes an array of battery cells, a DC/DC converter coupled to the battery cells, and a first switch logic coupled to the battery cells and the DC/DC converter. The first switch logic is configured to switch the BBU to operate between a first mode and a second mode. When operating in the first mode, the first switch logic causes the output voltage of the DC/DC converter to be provided to an external load. When operating in the second mode, the first switch logic causes the output voltage of the DC/DC converter to be coupled to an internal load for the purpose of determining the health of the battery cells.

TECHNICAL FIELD

Embodiments of the present disclosure relate generally to a backupbattery unit to provide backup power. More particularly, embodiments ofthe disclosure relate to a backup battery unit with a self-inspectioncircuit for determining health of the backup battery unit and othertesting purposes.

BACKGROUND

Battery energy storage as one of the energy storage methods issignificant important in various applications, such as, electricalvehicles (EVs), consumer electronics, micro grids, solar and wind power,and data center backup units. It provides essential energy to supportthe applications either as the only source or as a backup when the mainpower source is not available. Thus, it is extremely important toguarantee its availability and functionality.

In intelligent data centers, a battery backup unit (BBU) in electronicracks is an important device to provide alternative power to the serverwhen the main power supply is out of service. Lithium-ion batteries arethe most commonly used battery type for a BBU. However, because thebackup time of a BBU is relatively short (e.g., less than few minutes)and the discharging current is very high, the degradation of Li-ioncells will affect the available capacity. As a result, the backup timeduration with high current may not be satisfied. In order to guarantee asafe, reliable and high efficient operation condition, the failuredetection of the BBU is necessary. There has been a lack of efficientways to determine the health of battery cells, particularly, inelectronic racks of a data center.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the disclosure are illustrated by way of example and notlimitation in the figures of the accompanying drawings in which likereferences indicate similar elements.

FIG. 1 is a block diagram illustrating an example of a backup batteryunit according to one embodiment.

FIG. 2 is a block diagram illustrating an example of a backup batteryunit according to another embodiment.

FIG. 3 is a block diagram illustrating an example of a backup batteryunit according to another embodiment.

FIG. 4 shows an example of an electronic rack containing a backupbattery pack according to one embodiment.

DETAILED DESCRIPTION

Various embodiments and aspects of the disclosures will be describedwith reference to details discussed below, and the accompanying drawingswill illustrate the various embodiments. The following description anddrawings are illustrative of the disclosure and are not to be construedas limiting the disclosure. Numerous specific details are described toprovide a thorough understanding of various embodiments of the presentdisclosure. However, in certain instances, well-known or conventionaldetails are not described in order to provide a concise discussion ofembodiments of the present disclosures.

Reference in the specification to “one embodiment” or “an embodiment”means that a particular feature, structure, or characteristic describedin conjunction with the embodiment can be included in at least oneembodiment of the disclosure. The appearances of the phrase “in oneembodiment” in various places in the specification do not necessarilyall refer to the same embodiment.

According to some embodiments, a topology is proposed with differentmodes to enable various self-inspection operations, such as temperaturerise test, protection test, degradation test, and etc., withoutinterrupting normal operation. A switched-mode control is utilized for aBBU of a data center to perform the main power voltage regulation andbattery impedance detection. There are two control modes in the BBUcontrol system. In the first control mode, the main power voltage (alsoreferred to as a busbar voltage) is regulated at the desired referencevalue by regulating the duty cycle of a direct-current to direct-current(DC/DC) power converter. In the second control mode, the BBU is bypassedfrom the external load (e.g., the main power) and coupled to an internalload as a test load for the purpose of determining the health of theBBU. At the same time, a perturbation signal at a given frequency isadded to the duty cycle of the DC/DC converter to perturb voltage andcurrent of the battery cells in the BBU.

By detecting the magnitude and phase of battery voltage and current, theinternal impedance can be detected at the real-time to evaluate thehealth of the BBU. With the switched-mode control, only one DC/DC powerconverter is required to perform the main voltage regulation andimpedance detection, which reduces the volume, cost and size of the BBU.The operating modes of BBUs enable standalone inspection withoutinterfering the normal operations while increasing the reliability. Itdoes not require physically removing the BBU form an electronic rack inorder to perform the inspection. It only requires a very small amount ofenergy for the inspection, which may not require the battery cells to berecharged after the testing.

According to one aspect, a BBU includes an array of battery cells, aDC/DC converter coupled to the battery cells, and a first switch logiccoupled to the battery cells and the DC/DC converter. The first switchlogic is configured to switch the BBU to operate between a first modeand a second mode. When operating in the first mode, the first switchlogic causes the output voltage of the DC/DC converter to be provided toan external load. When operating in the second mode, the first switchlogic causes the output voltage of the DC/DC converter to be coupled toan internal load for the purpose of determining the health of thebattery cells.

In one embodiment, the battery cells are coupled in parallel when theyare coupled to the DC/DC converter. In one embodiment, the BBU furtherincludes a second switch logic coupled between the battery cells and theDC/DC converter to switch the battery cells to operate between the firstmode and the second mode. In one embodiment, the BBU further includes anarray of switching devices, one for each of the battery cells to couplea corresponding battery cell to the DC/DC converter respectively. Whenoperating in the first mode, the second switch logic is configured todirectly couple the battery cells to the DC/DC converter. When operatingin the second mode, the second switch logic is configured to couple thebattery cells to the DC/DC converter via their respective switchingdevices. In one embodiment, at least one of the switching devices is aunidirectional switching device such as a diode, which only allows acurrent flowing from a battery cell to the DC/DC converter. In oneembodiment, a diode is coupled between a positive terminal of a batterycell and a positive terminal of the DC/DC converter.

According to another aspect, an electronic rack includes an array ofserver blades, each including a computer server for data processing. Theelectronic rack further includes a power supply to provide power to theserver blades and a BBU to provide backup power to the server bladeswhen the power supply is unavailable. The BBU includes components thatcan operate in a first mode and a second mode as described above.

FIG. 1 is a schematic diagram illustrating an example of a BBU accordingto one embodiment. Referring to FIG. 1, BBU 100 may be a BBU insertedinto an electronic rack of a data center. In one embodiment, BBU 100includes an array of one or more battery cells 101A-101B (collectivelyreferred to as battery cells 101). Although there are only two batterycells, more or fewer numbers of battery cells may be applicable.

BBU 100 further includes a DC/DC converter 102 coupled to the batterycells 101 and a first switch logic 111 coupled to the battery cells 101and the DC/DC converter 102. The first switch logic 111 is configured toswitch BBU 100 to operate between a first mode and a second mode. Whenoperating in the first mode, the first switch logic 101 causes theoutput voltage of the DC/DC converter 102 to be provided to an externalload 103. That is, when operating in a first mode, switch logic 111 isswitched to a first position to cause the output of converter 102 to becoupled to external load 103. External load 103 may represent a computerserver or electronic device that draws current or power from converter102 (e.g., discharging of battery cells). Alternatively, external load103 may represent an external power supply (e.g., a rack power supply ofan electronic rack) that is configured to charge battery cells 101.

When operating in the second mode, the first switch logic 111 causes theoutput voltage of the DC/DC converter 102 to be coupled to an internalload 104 for the purpose of determining the health of the battery cells101. That is, when operating in the second mode, switch logic 111 isswitched to a second position to cause the output of DC/DC converter 102to be coupled to internal load 104, which in this example a resisterwith a predetermined resistance value. When switch logic 111 is switchedto the second position in the second mode, external load 103 isdecoupled from DC/DC converter 102. Similarly, when switch logic 111 isswitched to the first position in the first mode, internal load 104 isdecoupled from DC/DC converter 102.

In one embodiment, the battery cells 101 are coupled in parallel whenthey are coupled to the DC/DC converter 102. In one embodiment, BBU 100further includes a second switch logic 112 coupled between the batterycells 101 and the DC/DC converter 102 to switch the battery cells 101 tooperate between the first mode and the second mode. In one embodiment,BBU 100 further includes an array of switching devices 113A-113B(collectively referred to as switching devices 113), one for each of thebattery cells 101 to couple a corresponding battery cell to the DC/DCconverter 102 respectively.

When operating in the first mode, the second switch logic is switched toa first position to directly couple the battery cells 101 to the DC/DCconverter 102 via connections 114A-114B (collectively referred to asconnections 114). When operating in the second mode, the second switchlogic is switched to a second position to couple the battery cells 101to the DC/DC converter 102 via their respective switching devices113A-113B. In one embodiment, at least one of the switching devices113A-113B is a unidirectional switching device such as a diode as shownin FIG. 2, which only allows a current flowing from a battery cell tothe DC/DC converter 102. In one embodiment, a diode is coupled between apositive terminal of a battery cell and a positive terminal of the DC/DCconverter 102.

Referring to FIG. 2, in this example, the anode terminal of a diode iscoupled to a terminal of a battery cell and the cathode terminal of thediode is coupled to a terminal of DC/DC converter 102. Such a connectionallows an electrical current flowing in a single direction from thebattery cell towards DC/DC converter 102. As a result, during the secondmode, an electronic current is prevented from flowing backwardly fromDC/DC converter 102 into the battery cell. Similarly, the current isprevented from flowing backwardly from one battery cell to anotherbattery cell. In other words, one battery cell cannot charge anotherbattery cell.

Referring back to FIG. 1, in one embodiment, controller 110 isconfigured to generate control signals to switch logic 111-112 tocontrol switch logic 111-112 to cause the BBU 100 to operate between thefirst mode and the second mode. That is, controller 110 generates propercommands or signals to cause switch logic 111-112 to switch between thefirst position and the second position, which in turn causes BBU 110 toswitch between the first mode and second mode. For example, switch logic111-112 may be configured by controller 110 to synchronously switchbetween the first position and the second position, such that BBU 110 isto operate between the first mode and the second mode. Controller 110may generate commands or signals in response to a user input.Alternatively, controller 110 may operate automatically based on apredetermined schedule, such as a self-inspection or testing schedule,to periodically configure BBU 110 to operate in the second mode, suchthat an internal test or health determination can be performed.

In one embodiment, when BBU 100 is configured in the second mode, an SOH(state of health) or the health of battery cells is determined based onone or more parameters of the battery cells measured during the secondmode. For example, an internal impedance of a battery cell may bemeasured to determine the health of the battery cell. In addition, basedon the internal impedance of each battery cell, a load or energy balanceamongst the battery cells can be derived. The available battery capacitycan also be determined. Based on at least some of the above parameters,a charge time or discharge time of the BBU can be calculated. This isimportant when a BBU is utilized within an electronic rack of a datacenter. When the main power is unavailable, the BBU needs to providepower to at least allow the electronic rack to back up its data to asafe storage during the power outage. By determining the health of theBBU, the backup time period that the BBU can support may be determineahead of the power outage. If it is determined the BBU's health is notsufficient, the data processing task may be offloaded or migrated toanother electronic rack for safety reasons before it is too late.

FIG. 3 shows a BBU according to a particular embodiment with a built-inself-inspection mechanism as discussed above. Referring to FIG. 3, eachbattery cell/module is coupled in parallel at the input of a DC/DC powerconverter. In order to provide high current to the load/server, multipleBBUs may be coupled in parallel at the Busbar (e.g., the main power railof an electronic rack). As shown in FIG. 3, there are two control modesin the controller design. Mode I is the normal operation mode to performthe voltage regulation or current regulation for battery charging. ModeII is the health detection mode by measuring the battery AC impedance asthe health indicator. For most of the time, BBUs operate in the normaloperation mode. The Busbar voltage is sensed to compare with thereference value VBus_ref and the error is sent to the voltagecompensator. The output of voltage compensator is the duty cycle of theDC/DC converter.

In control Mode II, the connection is changed from busbar (which iscoupled with server) to a test load. As shown in FIG. 3, the duty cycleof the DC/DC converter is obtained by adding a sinusoidal perturbationto it. DC average value is given by Eq. (1) as follows.

d(t)=D _(avg) +D _(ptb)×sin(2πf _(ptb) t)   (1)

As a result, a small ripple over the DC/average values of the voltageand current of different batteries is generated and given by thefollowing equations.

$\quad\{ {\begin{matrix}{i_{{battery}\; 1} = {I_{{battery\_ avg}\mspace{11mu} 1} + {I_{{battery\_ ptb}1} \times {\sin ( {{2\; \pi \; f_{ptb}t} + \theta_{i\; 1}} )}}}} \\{i_{{battery}\; 2} = {I_{{battery\_ avg}\mspace{11mu} 2} + {I_{{battery\_ avg}\mspace{11mu} 2} \times {\sin ( {{2\; \pi \; f_{ptb}t} + \theta_{i\; 2}} )}}}} \\{i_{{battery}\; N} = {I_{{battery}\; N} + {I_{{battery}\; N} \times {\sin ( {{2\; \pi \; f_{ptb}t} + \theta_{i\; N}} )}}}}\end{matrix}{\quad\{ \begin{matrix}{v_{{battery}\; 1} = {V_{{battery\_ avg}\mspace{11mu} 1} + {V_{{battery\_ ptb}1} \times {\sin ( {{2\; \pi \; f_{ptb}t} + \theta_{v\; 1}} )}}}} \\{v_{{battery}\; 2} = {V_{{battery\_ avg}\mspace{11mu} 2} + {V_{{battery\_ avg}\mspace{11mu} 2} \times {\sin ( {{2\; \pi \; f_{ptb}t} + \theta_{v\; 2}} )}}}} \\{v_{{battery}\; N} = {V_{{battery}\; N} + {V_{{battery}\; N} \times {\sin ( {{2\; \pi \; f_{ptb}t} + \theta_{vN}} )}}}}\end{matrix} }} $

where Ibattery_avg and Vbattery_avg are the average values of batterycurrent and voltage; Ibattery_ptb and Vbattery_ptb are the perturbationvalues of battery current and voltage; fptb is the perturbationfrequency; and θnd θare the phase shift of battery current and voltagecompared with duty cycle.

By detecting the magnitude and phase of voltage and currentperturbation, the impedance of batteries can be calculated based on thefollowing equations.

${{Z_{battery}( f_{ptb} )}} = {{\frac{V_{battery\_ ptb}}{I_{battery\_ ptb}} < {Z_{battery}( f_{ptb} )}} = {\theta_{v} - \theta_{i}}}$

The impedance of battery can be used as the indicator to detect thebattery health situation.

As described above, a BBU can be utilized as a backup power supply unitin an electronic rack of a data center. An electronic rack includes anarray of server blades, each including a computer server for dataprocessing. The electronic rack further includes a power supply toprovide power to the server blades and a BBU to provide backup power tothe server blades when the power supply is unavailable. The BBU includescomponents that can operate in a first mode and a second mode asdescribed above. By embedding a self-inspection circuit as a part of theBBU, the health of the BBU can be determined, manually in response to auser input or automatically according to a maintenance schedule, withouthaving to physically remove the BBU from the electronic rack. Forexample, a user can simply push a button or turn on a switch of anelectronic rack to send a signal to a controller (e.g., controller 110)of a BBU, which in turns configure the BBU in the second mode (e.g.,health determination mode) and a variety of battery operating parameterscan be measured, which can be utilized to determine the SOH or thehealth of the BBU.

FIG. 4 is a block diagram illustrating an example of an electronic rackaccording to one embodiment. Electronic rack 900 may include one or moreserver slots to contain one or more servers respectively. Each serverincludes one or more information technology (IT) components (e.g.,processors, memory, storage devices, network interfaces). Referring toFIG. 4, according to one embodiment, electronic rack 900 includes, butis not limited to, CDU 901, rack management unit (RMU) 902 (optional), apower supply unit (PSU) 950, a BBU 910, and one or more server blades903A-903D (collectively referred to as server blades 903). Server blades903 can be inserted into an array of server slots respectively fromfrontend 904 or backend 905 of electronic rack 900. The PSU 950 and/orBBU 910 may be inserted into any of server slots 903 within theelectronic rack 900.

Note that although there are only four server blades 903A-903D shownhere, more or fewer server blades may be maintained within electronicrack 900. Also note that the particular positions of CDU 901, RMU 902,PSU 950, BBU 910, and server blades 903 are shown for the purpose ofillustration only; other arrangements or configurations of CDU 901, RMU902, BBU 910, and server blades 903 may also be implemented. Note thatelectronic rack 900 can be either open to the environment or partiallycontained by a rack container, as long as the cooling fans can generateairflows from the frontend to the backend.

In addition, a fan module can be associated with each of the serverblades 903, and BBU 910. In this embodiment, fan modules 931A-931E,collectively referred to as fan modules 931, and are associated withserver blades 903A-903D and BBU 910 respectively. Each of the fanmodules 931 includes one or more cooling fans. Fan modules 931 may bemounted on the backends of server blades 903 and BBU 910 to generateairflows flowing from frontend 904, traveling through the air space ofthe sever blades 903, and existing at backend 905 of electronic rack900.

In one embodiment, CDU 901 mainly includes heat exchanger 911, liquidpump 912, and a pump controller (not shown), and some other componentssuch as a liquid reservoir, a power supply, monitoring sensors and soon. Heat exchanger 911 may be a liquid-to-liquid heat exchanger. Heatexchanger 911 includes a first loop with inlet and outlet ports having afirst pair of liquid connectors coupled to external liquid supply/returnlines 931-932 to form a primary loop. The connectors coupled to theexternal liquid supply/return lines 931-932 may be disposed or mountedon backend 905 of electronic rack 900. The liquid supply/return lines931-932 are coupled to a set of room manifolds, which are coupled to anexternal heat removal system, or extremal cooling loop. In addition,heat exchanger 911 further includes a second loop with two ports havinga second pair of liquid connectors coupled to liquid manifold 925 toform a secondary loop, which may include a supply manifold to supplycooling liquid to server blades 903 and a return manifold to returnwarmer liquid back to CDU 901. Note that CDUs 901 can be any kind ofCDUs commercially available or customized ones. Thus, the details ofCDUs 901 will not be described herein. As an example, cooling device 108shown in FIG. 7 may connect to 925 to complete a full fluid loop.

Each of server blades 903 may include one or more IT components (e.g.,central processing units or CPUs, graphical processing units (GPUs),memory, and/or storage devices). Each IT component may perform dataprocessing tasks, where the IT component may include software installedin a storage device, loaded into the memory, and executed by one or moreprocessors to perform the data processing tasks. At least some of theseIT components may be attached to the bottom of any of the coolingdevices as described above. Server blades 903 may include a host server(referred to as a host node) coupled to one or more compute servers(also referred to as computing nodes, such as CPU server and GPUserver). The host server (having one or more CPUs) typically interfaceswith clients over a network (e.g., Internet) to receive a request for aparticular service such as storage services (e.g., cloud-based storageservices such as backup and/or restoration), executing an application toperform certain operations (e.g., image processing, deep data learningalgorithms or modeling, etc., as a part of a software-as-a-service orSaaS platform). In response to the request, the host server distributesthe tasks to one or more of the performance computing nodes or computeservers (having one or more GPUs) managed by the host server. Theperformance compute servers perform the actual tasks, which may generateheat during the operations.

Electronic rack 900 further includes optional RMU 902 configured toprovide and manage power supplied to servers 903, fan modules 931, andCDU 901. Optimization module 921 and RMC 922 can communicate with acontroller in some of the applications. RMU 902 may be coupled to powersupply unit 950 to manage the power consumption of the power supplyunit. The power supply unit 950 may include the necessary circuitry(e.g., an alternating current (AC) to direct current (DC) or DC to DCpower converter, backup battery, transformer, or regulator, etc.,) toprovide power to the rest of the components of electronic rack 900.

In one embodiment, RMU 902 includes optimization module 921 and rackmanagement controller (RMC) 922. RMC 922 may include a monitor tomonitor operating status of various components within electronic rack900, such as, for example, computing nodes 903, CDU 901, and fan modules931. Specifically, the monitor receives operating data from varioussensors representing the operating environments of electronic rack 900.For example, the monitor may receive operating data representingtemperatures of the processors, cooling liquid, and airflows, which maybe captured and collected via various temperature sensors. The monitormay also receive data representing the fan power and pump powergenerated by the fan modules 931 and liquid pump 912, which may beproportional to their respective speeds. These operating data arereferred to as real-time operating data. Note that the monitor may beimplemented as a separate module within RMU 902.

Based on the operating data, optimization module 921 performs anoptimization using a predetermined optimization function or optimizationmodel to derive a set of optimal fan speeds for fan modules 931 and anoptimal pump speed for liquid pump 912, such that the total powerconsumption of liquid pump 912 and fan modules 931 reaches minimum,while the operating data associated with liquid pump 912 and coolingfans of fan modules 931 are within their respective designedspecifications. Once the optimal pump speed and optimal fan speeds havebeen determined, RMC 922 configures liquid pump 912 and cooling fans offan modules 931 based on the optimal pump speed and fan speeds.

As an example, based on the optimal pump speed, RMC 922 communicateswith a pump controller of CDU 901 to control the speed of liquid pump912, which in turn controls a liquid flow rate of cooling liquidsupplied to the liquid manifold 925 to be distributed to at least someof server blades 903. Therefore, the operating condition and thecorresponding cooling device performance are adjusted. Similarly, basedon the optimal fan speeds, RMC 922 communicates with each of the fanmodules 931 to control the speed of each cooling fan of the fan modules931, which in turn control the airflow rates of the fan modules 931.Note that each of fan modules 931 may be individually controlled withits specific optimal fan speed, and different fan modules and/ordifferent cooling fans within the same fan module may have differentoptimal fan speeds. According to one embodiment, BBU 910 can beimplemented as any of the BBUs described above as shown in FIGS. 1-3.

Note that some or all of the IT components of servers 903 may beattached to any one of the cooling devices described above, either viaair cooling using a heatsink or via liquid cooling using a cold plate.One server may utilize air cooling while another server may utilizeliquid cooling. Alternatively, one IT component of a server may utilizeair cooling while another IT component of the same server may utilizeliquid cooling. In addition, a switch is not shown here, which can beeither air cooled or liquid cooled.

In the foregoing specification, embodiments of the disclosure have beendescribed with reference to specific exemplary embodiments thereof. Itwill be evident that various modifications may be made thereto withoutdeparting from the broader spirit and scope of the disclosure as setforth in the following claims. The specification and drawings are,accordingly, to be regarded in an illustrative sense rather than arestrictive sense.

What is claimed is:
 1. A backup battery unit (BBU) to provide backuppower, the BBU comprising: a plurality of battery cells; adirect-current to direct-current (DC/DC) converter coupled to thebattery cells to generate and regulate an output voltage based onbattery energy provided by the battery cells; and a first switch logiccoupled to the battery cells and the DC/DC converter, wherein the firstswitch logic is configured to switch the BBU to operate between a firstmode and a second mode, wherein when operating in the first mode, thefirst switch logic causes the output voltage to be provided to anexternal load, and wherein when operating in the second mode, the firstswitch logic causes the output voltage to be coupled to an internal loadfor determining health of the battery cells.
 2. The BBU of claim 1,wherein the plurality of battery cells are coupled in parallel to theDC/DC converter.
 3. The BBU of claim 2, further comprising a secondswitch logic coupled between the battery cells and the DC/DC converterto switch the battery cells to operate between the first mode and thesecond mode.
 4. The BBU of claim 3, wherein the second switch logiccomprises a plurality of switching devices, one for each of the batterycells to couple a corresponding battery cell to the DC/DC converterrespectively.
 5. The BBU of claim 4, wherein when operating in the firstmode, the second switch logic is configured to directly couple thebattery cells to the DC/DC converter, and wherein when operating in thesecond mode, the second switch logic is configured to couple the batterycells to the DC/DC via their respective switching devices.
 6. The BBU ofclaim 5, wherein each of the switching devices is a unidirectionalswitching device that only allows an electronic current flowing from thecorresponding battery cell to the DC/DC converter.
 7. The BBU of claim6, wherein at least one of the switching devices comprises a diode. 8.The BBU of claim 6, wherein each of the switching devices is coupled inseries between a positive terminal of the corresponding battery cell toa positive terminal of the DC/DC converter.
 9. The BBU of claim 1,wherein the DC/DC converter is a bidirectional DC/DC converter, andwherein the external load represents a computer server of an electronicrack or an external power supply.
 10. The BBU of claim 9, wherein whenthe external load is an external power supply, the external power supplyis configured to provide power to charge the battery cells.
 11. Anelectronic rack of a data center, comprising: a plurality of serverblades arranged in a stack, each server blade including one or moreservers to provide data processing services; a power supply coupled tothe server blades to provide power to operate the servers; and a backupbattery unit (BBU) coupled to the server blades to provide backup powerto the servers when the power supply is unable to provide power, whereinthe BBU comprises a plurality of battery cells, a direct-current todirect-current (DC/DC) converter coupled to the battery cells togenerate and regulate an output voltage based on battery energy providedby the battery cells, and a first switch logic coupled to the batterycells and the DC/DC converter, wherein the first switch logic isconfigured to switch the BBU to operate between a first mode and asecond mode, wherein when operating in the first mode, the first switchlogic causes the output voltage to be provided to an external load, andwherein when operating in the second mode, the first switch logic causesthe output voltage to be coupled to an internal load for determininghealth of the battery cells.
 12. The electronic rack of claim 11,wherein the plurality of battery cells are coupled in parallel to theDC/DC converter.
 13. The electronic rack of claim 12, wherein the BBUfurther comprises a second switch logic coupled between the batterycells and the DC/DC converter to switch the battery cells to operatebetween the first mode and the second mode.
 14. The electronic rack ofclaim 13, wherein the second switch logic comprises a plurality ofswitching devices, one for each of the battery cells to couple acorresponding battery cell to the DC/DC converter respectively.
 15. Theelectronic rack of claim 14, wherein when operating in the first mode,the second switch logic is configured to directly couple the batterycells to the DC/DC converter, and wherein when operating in the secondmode, the second switch logic is configured to couple the battery cellsto the DC/DC via their respective switching devices.
 16. The electronicrack of claim 15, wherein each of the switching devices is aunidirectional switching device that only allows an electronic currentflowing from the corresponding battery cell to the DC/DC converter. 17.The electronic rack of claim 16, wherein at least one of the switchingdevices comprises a diode.
 18. The electronic rack of claim 16, whereineach of the switching devices is coupled in series between a positiveterminal of the corresponding battery cell to a positive terminal of theDC/DC converter.
 19. The electronic rack of claim 11, wherein the DC/DCconverter is a bidirectional DC/DC converter, and wherein the externalload represents a computer server of an electronic rack or an externalpower supply.
 20. The electronic rack of claim 19, wherein when theexternal load is an external power supply, the external power supply isconfigured to provide power to charge the battery cells.